This invention relates to a high-performance rubber composition which combines excellent wear resistance with low heat buildup and is suited for use in tread members of large-size radial tires for trucks and buses.
Carbon black for the reinforcement of rubber is classified into many types according to the properties of their own. Each of these properties is a major determinant of the characteristics of a given rubber composition in which the particular carbon black is incorporated. Usually, therefore, when a carbon black is to be compounded into rubber, one capable of imparting the properties suited for the intended use of the resulting rubber goods is chosen.
Rubber members, e.g., the treads of large tires for trucks and buses, are required to be highly abrasion-resistant under severe service conditions. For these applications it has been believed effective to use a carbon black of small particle size and large specific surface area, such as SAF (N110) or ISAF (N220).
However, such a hard carbon black tends to increase the heat buildup of rubber goods. When used in tire treads, it can promote heat buildup in running tires, eventually destroying the inside structure or causing premature aging of the tire tread structural material.
Meanwhile, development of fuel-saving tires is energetically under way as part of efforts to meet the public needs for the reduction of natural resource and energy consumption. For the development of such economical tires a rubber composition of low heat build-up is indispensable.
It follows that an ideal rubber member for the tread of those tires could be obtained if both high abrasion resistance and low heat buildup could be imparted to the rubber member using a carbon black of small particle size and large specific surface area.
As regards the rubber compositions that could impart both of the contradictory properties, high abrasion resistance and low heat buildup, systematic investigations have been made from the standpoint of carbon black. They are typified by the following proposals (a) to (c).
(a) A rubber composition for large tire treads containing a carbon black which is fine in particle size and yet has a relatively broad aggregates Strokes diameter distribution (Japanese patent application Kokai publication No. 63-112638).
(b) A rubber composition containing a carbon black which has two maximum points within a specific range of the aggregates Stokes diameter distribution (Japanese patent application Kokai publication No. 63-179941).
(c) A rubber composition containing a carbon black which has a value defined by a formula within a specific range, the formula using as variables the dibutyl phthalate absorption number, dibutyl phthalate absorption number of compressed sample, blackness, nitrogen adsorption specific surface area, and iodine adsorption (Japanese patent application Kokai publication No. 63-297439).
However, the composition (a) is not fully satisfactory in respect of abrasion resistance, although the heat buildup of the resulting rubber article is low on account of the broad aggregate distribution of the carbon black.
The composition (b) gives the rubber article both low heat buildup and high abrasion resistance but it fails to satisfy the market requirements that are becoming more and more severe.
The same applies to the composition (c) and the latter is yet to be improved to meet the market needs.
The present invention has been accomplished in the course of development of a rubber composition in the wake of the compositions (a) to (c) above. It is predicated upon the discovery that rubber properties suitable for the treads of large tires are obtained by mixing a rubber component with a carbon black which has a true specific gravity and a mode diameter of intraaggregate pore each within a specific range.